A wireless communication system communicates between a communication device such as a mobile station (MS) and a base station (BS). The MS can communicates by radio with the BS in the wireless coverage zone (cell or sector) provided by the BS.
The MS may belong to two or more wireless coverage zones and can receive individual signals from two or more BSs. In this case, the MS undergoes an increase in interference of received signals in the case where a plurality of BSs cannot transmit and receive signals in precise synchronization with respective predetermined timings (that is, the synchronization is not established between the BSs). The occurrence and increase of this interferences cause a decrease in efficiency of frequency usage in the wireless communication system.
In order that the MS can stably communicate with a plurality of BSs in their overlapped wireless coverage zone, how to establish the synchronization between the BSs is important.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2008-5003
[Patent Document 2] Japanese Laid-open Patent Publication No. 2008-5004
For example, a method of establishing synchronization between BSs using a global positioning system (GPS) receiver is known. This method requires each of a plurality of BSs to have a GPS receiver. Each BS generates a reference clock signal on the basis of signals from GPS satellites received by the GPS receiver, and performs transmission and reception of signals from/to a MS in synchronization with the reference clock signal. This allows the plurality of BSs to establish synchronization.
However, this method additionally requires antennas to receive signals from GPS satellites, and may expand the scale of the BSs. Besides, a GPS receiver provided for every BS leads to an increase in cost for system infrastructure.
A known method that does not use GPS establishes synchronization through wireless communication between BSs.
In order to ensure establishment of synchronization between BSs by this method, the BSs must be located somewhat closely. Accordingly, the wireless coverage zones for a MS are restricted. Besides, the setup of a communication time for synchronization control between the BSs restricts the communication time between the BSs and the MS. This causes a decrease in throughput of the wireless communication system.